Aluminum cold plates and plate-fin heat exchangers are commonly used for liquid cooling in military and aerospace applications because of their light weight and high performance. There are many factors to consider when designing these cold plates and heat exchangers: thermal performance, size, fluid pressure drop, weight, burst pressure, and corrosion resistance. This article will address the corrosion resistance considerations in designing, building, and operating aluminum cold plates and plate-fin heat exchangers.
Aluminum is known for its corrosion resistance. Under the right conditions, aluminum rapidly forms a protective oxide layer. Generally this occurs when oxygen is readily available and the surrounding medium has a moderate pH. There are two typical manifestations of aluminum corrosion: uniform corrosion and local corrosion. Uniform corrosion happens when the oxide layer is soluble in the corrosive medium.1 "The oxide film is soluble in alkaline solutions and in strong acids…but is stable over a pH range of approximately 4.0 - 9.0."2 In uniform corrosion, the entire oxide layer is being stripped away faster than it can reform. Local corrosion, usually in the form of a pit, occurs when there is non-uniformity in the base metal or the surrounding environment. The metal may have a local concentration of alloying elements that creates a galvanic couple. Similarly, the surrounding environment may have a local concentration of active elements such as chlorides.
Cold plates and heat exchangers are used with many different fluids and typically involve the recirculation of the same fluid. One fluid that should not be used in aluminum cold plates and heat exchangers is water. Tap water, in particular, can contain active ions, such as copper, bicarbonates, chlorides, and/or other impurities that facilitate corrosion. Plus, over time, the recirculation of the same fluid in a closed loop will cause the dissolved oxygen to come out of solution. The resulting lack of oxygen will inhibit the formation of the oxide layer. Given enough time, aluminum will eventually corrode if isolated from oxygen and exposed to low quality water.
When water is the preferred choice for a heat transfer system, distilled water is usually combined with a glycol to reduce the freezing point and increase the boiling point. For the reasons stated above, it is critical that corrosion inhibitors be used. Corrosion inhibitors are controlled amounts of active ions (usually phosphates) that take over the role of oxygen in forming a corrosion resistant layer. Since these inhibitors depend on a chemical reaction with the aluminum, using low quality water such as tap water would reduce the inhibitors' effectiveness.
Alloy selection is a key factor in high corrosion resistance. For example, braze sheets, which separate the fluid passages in plate-fin heat exchangers, consist of an internal core and external clad layer that usually represents about 10% of the overall sheet thickness. The clad layer is a brazing alloy that joins the braze sheet to both the hot and cold fins and the braze sheet to the side bars. Vacuum brazing alloys use silicone and other elements to lower the melting point of the alloy. Since the braze alloy is more anodic than the core, the braze alloy provides cathodic protection, and thus corrosion protection. Cathodic protection is a concept that has been used in the ship building business for decades. For hulls made of steel, a plug made of an active element, like zinc, is used to protect the hull. Because zinc is more active than steel, the zinc corrodes faster that the steel. Among the alloying elements of aluminum, the alloys with a minimum of copper and iron have the best corrosion resistance.3 "3xxx series alloys are generally among those having the highest general corrosion resistance…The 6xxx alloys also have high resistance."4
There are other considerations in cold plate and heat exchanger design. Internal fluid static pressure and external stresses put the core components under stress. These stresses often require that high strength alloys (6xxx series) be used for braze sheets and/or fins. Braze sheet thickness is a tradeoff between performance, weight, and corrosion protection. A thick braze sheet is heavy and reduces thermal performance. A thin braze sheet has less strength to withstand stresses and offers less corrosion protection. If a corrosive environment is present, thin braze sheets will withstand an attack for less time than a thicker sheet.
During their manufacturing process, cold plates and heat exchangers may be hydraulically tested with pure water. However, water should not remain in the unit longer than is needed to conduct the testing. A thorough drying process is critical to eliminate the possibility of water corrosion. "Bubble testing," or pressurizing a unit with a gas and submerging it in water, is used widely throughout the industry. This practice requires that the external surfaces be dried after testing.
When operating a water/glycol cold plate or heat exchanger, it is important to have a maintenance plan. The typical maintenance activity is flushing and refilling the system with the proper mixture of inhibited ethylene glycol and water. This should be done on a periodic basis at an interval determined through system level testing during the operational evaluation phase. Periodically, the fluid pH and refractive index should be measured. These measurements will change over time. From these measurements, a flushing frequency could be determined.
During deployment, it is common for coolant systems to be "topped off". This practice should not harm the cold plate or heat exchanger as long as the glycol concentration is not diluted to the point of making the inhibitor ineffective. Inhibitor effectiveness is a function of top-off water quality, other metal types in the fluid loop, and the age of inhibitor in the system. If "topping off" is employed, it is advisable to monitor the pH of the fluid. If the pH falls below 4.0 or rises above 9.0, a system flush/fill should take place as soon as possible.
Corrosion resistance begins with the cold plate's or heat exchanger's design. By working with a manufacturer that understands corrosion and how to prevent it, you'll achieve a better product. It is also important to develop maintenance procedures that will maximize the life of the aluminum cold plate or heat exchanger.
1 Editor J.R. Davis, ASM Metals Handbook, Desk Edition, Volume 2, Materials Park, OH, 1998, page 501.
2Ibid, page 499.
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